Mobile tracking units such as telematics devices may be used to track mobile assets such as vehicles (e.g., motor vehicles in a fleet), trailers, freight rail cars, shipping containers, and the like. Typically, a telematics device includes a navigation set, such as a Global Positioning System (GPS) receiver or other suitable navigation set, responsive to navigation signals transmitted by a set of navigation stations which can be either space- or earth-based. In each case, the navigation set is capable of providing data indicative of the mobile asset location based on the navigation signals. In addition, the telematics device can include a suitable communications system that may, for example, comprise an electromagnetic emitter or transceiver for transmitting to a remote location the mobile asset position data and other data acquired with sensing elements in the mobile asset.
While telematics devices typically include an internal power source such as a small rechargeable battery, telematics devices often are deployed in conditions that afford limited or intermittent access to an external power source for operating the device or recharging the battery. For example, in a telematics device affixed to a trailer, the internal battery may be able to receive a charge from an electrical connection to an auxiliary power source on an attached tractor, but will not receive any charge when the trailer is not attached to a tractor (e.g., when the trailer has been delivered to or deposited at a destination).
In some telematics devices, a battery can be supplemented by an array of solar panels arranged on an exterior surface of the asset, and configured to recharge the battery. However, a solar power source may often be unable to fully recharge the battery for any of numerous reasons, such as absence of sunlight, suboptimal weather conditions, dust or debris on the solar panels, or when the asset is parked wholly or partially in shade or under cover. Other supplemental external power sources, such as wind or piezoelectric power sources, are also unable to provide continuous power when the asset is not in motion.
The unavailability of continuous external power presents challenges in achieving efficient power management. Mobile tracking units used in a power-starved environment must be substantially power efficient in order to provide reliable and economical operation. Since both the navigation set and the communications system comprise devices which, when energized, generally consume a large portion of the overall electrical power consumed by the mobile tracking unit, it is desirable to control the timing of when such devices are activated or deactivated, so as to reduce the overall power consumption of the mobile tracking unit, while retaining acceptable performance in tracking of the mobile asset.
Some existing telematics devices are able to conserve power by entering a low-power mode (sometimes known as a sleep mode or hibernating mode) during periods when no external power source is available, and to wake up at fixed intervals to permit tracking. However, a fixed wake interval does not adequately address the perceived need to locate a mobile asset promptly on demand (e.g., immediately upon polling the telematics device), because the device will often be in sleep mode when it is polled.
In other telematics devices, when the battery charge reaches zero or falls below a predetermined level, the device may simply turn off without notice. Such an outcome is undesirable, especially when tracking mobile assets that contain perishable or valuable cargo.